Hyperpolarizabilities of substituted conjugated molecules. III. Study of a family of donor-acceptor disubstituted phenylpolyenes

Abstract

In Parts I and II of this series, we have made use of a finite field perturbated INDO scheme to compute molecular hyperpolarizabilities, to discriminate through simple symmetry considerations between respective σ and π electronic contributions, and to test these ideas on smaller size benzene monosubstitutes. Following the same approach, we report here investigation of a family of disubstituted donor-acceptor benzenes of increasing molecular size [NH2-C6H4-(CH = CH) n-NO2, with n = 0,1, and 2] and of great experimental interest in nonlinear optics (second harmonic generation). Supposedly responsible for the higher value of the second order hyperpolarizability (tensor βijk) of this type of molecules, the charge transfer process is explored here and its importance confirmed. Results are compared to experimental data with the help of a simple "two level" charge transfer model to account for frequency dispersion and found in satisfactory agreement wiith previously noted tendencies. Also confirmed here are the chain length influence on βijk and the prevailing part played by π electrons in second order nonlinear processes of conjugated molecules. This study is seen to bring a theoretical credit to the ever growing experimental interest in large charge transfer conjugated molecules in the field of nonlinear optics material research. © 1979 American Institute of Physics.